In digital radio broadcasts, signals are encoded in the digital domain, as opposed to traditional analog broadcasts that use amplitude modulated (AM) or frequency modulated (FM) techniques. The received and decoded digital audio signals have a number of advantages over their analog counterparts, such as a better sound quality, and a better robustness to radio interferences, such as multi-path interference, co-channel noise, etc. Several digital radio broadcast systems that have been developed and deployed, such as the Eureka 147 digital audio broadcasting (DAB) system and the in-band on-channel (IBOC) DAB system.
Many radio stations that transmit digital radio also transmit the same radio programme in an analog manner, for example using traditional amplitude modulated (AM) or frequency modulated (FM) transmissions. When two broadcasts for the same radio programme are available (e.g., either two digital broadcasts, or one digital and one analog broadcast, of the same programme), there is the possibility that the radio receiver may switch or cross-fade from one broadcast to the other, particularly when the reception of one is worse than that of the other. Examples of such switching strategies, often referred to as ‘blending’, are described in U.S. Pat. No. 6,590,944 and US publ. No. 2007/0291876.
When a blending operation from one broadcast technique to another broadcast technique is performed, it is known that artefacts may appear during a cross-fade, if the signals are not perfectly aligned. For example, if there is a small delay between the signals, they will exhibit opposite phases at particular frequencies, and these frequencies will be cancelled out at some point during the cross-fade. This happens even if the delay is as small as two samples.
Furthermore, it is difficult to calculate delays between the signal samples accurately in such real-time systems, in order to determine and correct artifacts due to slightly mis-aligned broadcast signals, particularly if computational resources are restricted. In addition, computing of accurate sampling delay is especially difficult if the signals have different characteristics, e.g., because different pre-processing has been applied. During the cross-fade, there can also be signal cancellation due to phase inversion (i.e., the signals having opposite phase). Next to this, one of the signals may have undergone processing with non-linear phase (e.g., filtering with an infinite impulse response filter), which makes the delay between the signals frequency dependent, and makes it practically impossible to adapt the signals to be perfectly aligned.
Thus, an improved audio processing circuit, audio unit, integrated circuit and method of spectrum blending in an audio unit is needed.